>Write everything twice quickly becomes write everything 4 times once a new change appears, just as quickly as it becomes write everything 8 times, and so on.

That's a good problem to have. Getting to 4 or 8 or 12, and then pruning it to 1 or maybe 2 or 3 clearly different cases, is better than shoehorning multiple cases into the wrong abstraction, having everything that speaks with them coupled to that and dancing around their assumptions, and then having to untangle that.

Duplicated code is by definition LESS coupled.

In C I used to make it so my standard per-file and per lib code could be cut and pasted to other files/libs without modification. (E.g. every file had a mLocal variable that was file-visibility symbols, every module had a #module define for logging, there was always a mLocal.stats member, etc. ) I think some of this duplicate vs. abstract depends on your languages expressiveness - Rust or Lisp with good compile type power make it possible to squeeze out a lot of duplication that in less expressive languages are just idioms - here’s the five lines to make a syscall, or here’s the skeleton of parsing a portable network buffer into a native object.

Having a lot of if/else in your code is definitely a cost. My weakness isn’t so much the libraries and APIs, but the actual binary - once I have a service that does A very well, and I run into needing A’ I mostly just add in a config line “op_mode = A|A’” and have the else/if chains in the server driving code. Moreso for CLIs that I use myself than production services, but I have added tunables for consistency and replication to datastores to allow new use cases and expand my footprint in the data center.